1. Field of the Invention
The present invention relates to a common-rail fuel-injection system for an internal combustion engine in which fuel forced from a high-pressure fuel-supply plunger pump is maintained at a constant pressure in a common rail, and is injected out of injectors connected with the common rail.
2. Description of the Prior Art
Common-rail fuel-injection systems have been conventionally known as the most suitable way to increase injection pressures and also control injection factors such as injection timing, amount of fuel injected per cycle and the like, depending on engine operating conditions. Among the prior common-rail fuel-injection systems there is a fuel-injection system in which working fluid pumped up to a preselected pressure is stored for fuel injection to actuate injectors, each of which is arranged in individual cylinders, thereby injecting a metered amount of fuel out of the injectors into their associated combustion chambers. A control unit controls valves installed in the individual injectors to inject fuel with the fuel-injection factors optimal to the engine operating conditions.
In contrast, the common-rail fuel-injection system of fuel-pressure actuated type has been known in which fuel serves as the working fluid. Governing for this type of fuel injection is effected by fuel pressures corresponding to the injection pressures, which are continually maintained in fuel passages from the common rail through injection lines to injection orifices formed at the distal ends of the injectors, each of which has a control valve allowing to flow or blocking the fuel supplied through the injection lines, and a solenoid-operated actuator to drive the control valve. A control unit regulates the fuel pressures in the common rail and the operation of the solenoid-operated actuators in the injectors to inject the pressurized fuel out of the individual injectors in accordance with the injection factors most suitable for the engine operating conditions. Moreover, a further another type of the common-rail fuel-injection system has been proposed, in which the working fluid is provided by engine oil stored under pressure in the common rail. The engine oil applied to pressure chambers in the injectors from the common rail provides hydraulic pressures to boost the fuel in pressure to a desired pressure, which is supplied into intensifying chambers in the injectors.
Referring to FIG. 7, the prior common-rail fuel-injection system of fuel-pressure actuated type will be explained in detail hereinafter.
Fuel drawn in by a fuel-feed pump 6 from a fuel tank 7 is applied to a high-pressure fuel-supply pump 1, which is a variable-delivery high-pressure plunger pump to force the fuel into a common rail 2. The fuel stored under pressure in the common rail 2 is allowed to pass through injection lines 23 included in a fuel passage system to injectors 3, each of which is installed in each cylinder, in accordance with the type of engine. The fuel finally is injected out of the individual injectors into their associated combustion chambers. The high-pressure fuel-supply plunger pump 1, besides the type illustrated, may be any one of rotary-plunger pump and inline-plunger pump in accordance with the type of engine.
The high-pressure fuel-supply plunger pump 1 has a cam 10 driven by the engine output to operate the pump, and a plunger 11 riding on the cam 10 to move in and out, with the plunger 11 forming at its top surface a part of the inside barrel wall defining a pumping chamber 12. An inlet valve 15 is arranged between the pumping chamber 12 and a fuel inlet line 13, and acts to regulate an amount of fuel forced into the pumping chamber 12 from the fuel-feed pump 6 through the fuel inlet line 13. A non-return valve 17 is interposed along a fuel discharge line 14 connecting the pumping chamber 12 with the common rail 2, and may open when the pressure created by the high-pressure fuel-supply plunger pump 1 is become over a preselected delivery pressure.
In order to keep the common-rail pressure from unexpected rise due to, for example, abnormality in control system, there is a relief valve 20, normally closed, which may open when subjected to a higher pressure than a preselected pressure, permitting the fuel held in the common rail 2 to escape to the fuel tank 7 through a relief line 21 with the result of reducing the common-rail pressure. Moreover, a pressure sensor 22 monitors the common-rail pressure Pr, which is in turn signaled to a control unit 8 of electronic controlled module, which is commonly contracted to EMC.
The injectors 3 are hermetically fitted with sealing members in holes bored in a base member such as a cylinder head. The injectors 3 each comprise a needle valve 31 movable up and down in a injector body, injection orifices 32 formed at an distal end of an injection nozzle to open when the needle valve 31 lifts off its seat, thereby injecting the fuel into a combustion chamber, not shown. The needle valve 31 has a top surface 33 that provides a part of a balance chamber 30, which is applied with the high-pressure fuel from the associated injection line 23. A fuel passage 34 connected with the injection line 23 is opened to a fuel sac 35 formed around the needle valve 31. Thus, the needle valve 31 exposed to the fuel sac 35 is subject to the fuel pressure at its first tapered surface 36, thus encountering the hydraulic force to lift the needle valve 31. On the other hand, the needle valve 31 encounters both of the downward thrust due to the fuel pressure in the balance chamber 30 and the return force of a return spring 47. Thus, the balance among the upward and downward hydraulic forces and the return force may govern the lift of the needle valve 31. On the closure of the needle valve 31, a second tapered surface 37 nearby the distal end of the needle valve 31 comes in engagement with a tapered valve seat to block the fuel passage between the injection orifices 32 and the fuel sac 35 around the needle valve 31.
While the high-pressure fuel in the common rail 2 is supplied to the balance chamber 30 through a fuel supply line 38 branching off from the injection line 23, the fuel in the balance chamber 30 is expelled through a drain line 40. The fuel supply line 38 and drain line 40 are provided respectively with throats 39, 41, that are defined such that the throat 41 is larger in effective cross-section area than another throat 39. Moreover, the drain line 40 is provided therein with a valve 44, which is to relieve the fuel in the drain line 40 to a fuel return line 46.
Control current from the control unit 8 energizes a solenoid 45 to open the valve 44 in the drain line 40. Thus, since the fuel flow at the throat 39 is more restricted than at the throat 41, the fuel pressure in the balance chamber 30 drops so that the force to lift the needle valve 31 off the seat overcomes the sum of the depressing force resulting from the fuel pressure in the balance chamber 30 and the resilient force of the return spring 47 to allow the needle valve 31 lifting off the seat with the fuel being injected out of the injection orifices 32 into the combustion chamber. The unconsumed fuel remaining the injector may be expelled out of the balance chamber 30 through the drain line 40 and recovered into the fuel tank 7 through the fuel return line 46.
The control unit 8 is applied with various signals of sensors 9 such as a crankshaft position sensor for detecting the engine rpm Ne, an accelerator pedal sensor for detecting the depression Ac of an accelerator pedal, and so on. The sensors 9 may also include other sensors for monitoring the engine operating conditions, for example, an engine coolant temperature sensor, a engine cylinder identifying sensor, a top dead center detection sensor, an atmospheric temperature sensor, an atmospheric pressure sensor, an intake manifold pressure sensor, and so on.
The control unit 8, on the basis of an injection characteristics map stored previously in memory, finds desired injection factors in accordance with the signals issued from the diverse sensors 9, and the valve 44 opens and closes, depending on the desired injection factors, to control the lift of the needle valve 31. The desired injection factors are defined to determine a injection timing and an amount of fuel injectes out of the injector 3 per cycle so as to make the engine output optimum for the engine operating conditions. The injection timing and the amount of fuel injected are dependent upon injection pressure as well as the lift, or amount and duration of lift, of the needle valve 31. The control unit 8 issues a command pulse to determine a driving current to energize the solenoid 45, which in turn opens and closes the valve 44 to regulate the lift of the needle valve 31.
Especially, the relation between the amount of fuel injected out of the injector 3 and the pulse width of the command pulse issued from the control unit 8 is plotted with the common rail pressure Pr, or fuel pressure in the common rail 2, as a parameter. The injection timing may be controlled by governing the time the command pulse is turned on/off, because the fuel injection starts or ceases with a preselected delay time after a time either the command pulse falls or rises. Relation between the fundamental amount of fuel injected and the engine rpm Ne is stored previously in a map of fundamental amount characteristics of fuel injected, in which they are plotted with the accelerator-pedal depression Ac as a parameter. Thus, the amount of fuel injected may be calculated on the basis of the map of fundamental amount characteristics of fuel injected, depending on the engine operating conditions. Although but only one injector 3 is shown in the illustrative example, the engine of this type is usually a multi-cylinder engine, for example, a four-cylinder engine or six-cylinder engine, and the control unit 8 controls individually the fuel injection for the injector 3 located in each cylinder.
As the injection pressure to inject the fuel out of the injector 3 is substantially equal the fuel pressure held in the common rail 2, the control of the common-rail pressure Pr results in controlling the injection pressure. Even if the engine operating conditions were held unvaried, the common-rail pressure Pr would drop due to fuel consumption in the common rail 2 at every fuel injection out of the injector 3. In contrast, when the engine operating conditions change, the common-rail pressure Pr should be altered to other common-rail pressure optimum for the changed operating conditions. To cope with this, the control unit 8 regulates amount delivered out of the high-pressure fuel-supply plunger pump 1 to keep the fuel pressure in the common to rail 2 at the preselected pressure or change continually it to the pressure required for the varied engine operating conditions.
For regulating the common-rail pressure Pr, a desired common-rail pressure is first found dependent on engine rpm Ne and the desired amount of fuel to be injected, which is determined in accordance with the engine operating conditions. Then, the amount of fuel delivered out of the high-pressure fuel-supply plunger pump 1, or the amount of fuel corresponding to the effective stroke of the plunger, is subjected to the feedback control to eliminate the deviation of actual common-rail pressure detected at the pressure sensor 22 from the desired common-rail pressure.
Among prior systems to regulate the amount of fuel delivered out of the high-pressure fuel-supply plunger pump 1 in the common-rail fuel-injection system shown in FIG. 7, there has been a system that is termed pre-stroke control, in which an inlet valve 15 is controlled according to the pre-stroke way. That is to say, the fuel admitted in the pumping chamber 12, although but allowed to return through the fuel inlet line 13 to the fuel tank 7 as long as a fuel inlet valve 15 in the fuel inlet line 13 is kept open, even during the lift stroke of the plunger 11, is forced towards the delivery side of the pump just after the inlet valve 15 has been closed whereby the amount of fuel delivered out of the high-pressure fuel-supply plunger pump 1 is regulated to result in controlling the common-rail pressure Pr. As there is provided a relief valve 18 to set an upper limit on the fuel pressure, or feed pressure, in the inlet line 13, excess fuel fed from the fuel-feed pump 6 is left returned through the relief valve 18 and return fuel return line 19 to the fuel tank 7.
Incidentally, the high-pressure fuel-supply pump is usually of the type having plural plungers regardless of either inline arrangement or rotary arrangement. In the prior common-rail fuel-injection systems, moreover, the actual common-rail pressure adopted to control the common-rail pressure is the value found to have on the average the common-rail pressures sensed at every fuel delivery of the individual plungers or at every fuel injection of the individual injectors. On the other hand, the amounts of fuel delivered at the individual plunger strokes and/or injected out of the individual injectors are hardly avoidable variations caused by the scattering in mechanical characteristics and aging of the individuals. Thus, as long as the amount of fuel to be delivered out of the high-pressure fuel-supply plunger pump is metered depending on the actual common-rail pressure represented by the average value as described just above, the feedback control seeking to keep the actual common-rail pressure the desired common-rail pressure results in causing the variations at every cylinder in either of the pressure rise in the common-rail pressure owing to the fuel delivery from the high-pressure fuel-supply plunger pump and the pressure drop in the common-rail pressure resulting from the fuel injection out of the injectors. This causes the scattering in fuel-injection pressure. In the common-rail fuel-injection system, it would be substantially impossible to make the fuel injected out at every injector the same amount so long as the variations in common-rail pressure are eliminated, even if the injectors were made uniform in their injection characteristics. On low-speed operation of engines such as idling, especially, scattering in injection at every cylinder raises the variations in combustion condition, which result in the occurrence of uncomfortable vibration or noise. In addition, even if the high-pressure fuel-supply plunger pump is of the type having a single plunger driven with a multi-lobe cam, there is the problem, as in the multi-plunger type described above, in which the amount of fuel delivered might vary at every cam-operation of the lobes.
Considering that the desired value of common-rail pressure to be determined dependent on the engine operating conditions should be the common-rail pressure at any timing of restoration in pressure, which is after the fuel has been delivered out of the high-pressure fuel-supply plunger pump into the common rail to recover the common-rail pressure but before the fuel injection at the individual injector is initiated to cause the pressure drop in the common rail, it will be expected to make the common-rail pressure restore uniformly to the desired value with the fuel discharge from the high-pressure fuel-supply plunger pump just after the fuel injection of the individual injector, regardless of whether or not there exists any scattering in the amount of fuel delivered out of the individual plunger pump.